Metallic alloys that are amorphous or glassy at low temperatures have been known in the prior art for a number of years. Amorphous alloys differ from ordinary metals in that these materials can be undercooled and remain as an extremely viscous liquid phase or glass at ambient temperatures when cooled sufficiently rapidly, whereas ordinary metals crystallize when cooled from the liquid phase.
Because metals naturally tend toward crystalline structures, the formation of amorphous metallic alloys has always faced the difficulty that the undercooled alloy melt tends toward crystallization. In short, to form an amorphous solid alloy one must coot a molten starting material from the melting temperature to below the glass transition temperature as quickly as possible to avoid crystallizing the metal. As a result, initial efforts to make amorphous alloys focused on a broad range of compositions that would form amorphous alloys when cooled at rates on the order of 104 to 106 K/sec. To achieve such rapid cooling rates, a very thin layer (e.g., on the order of 10s to 100s of micrometers) or small droplets of molten metal were brought into contact with a conductive substrate maintained at near ambient temperature. For example, early amorphous alloys were made by melt-spinning onto a cooled substrate, thin layer casting on a cooled substrate moving past a narrow nozzle, or by “splat quenching” droplets between cooled substrates. That these techniques were favored is the result of the need to extract heat at a sufficient rate to suppress crystallization, but as a consequence of these techniques early amorphous alloys were only available as ribbons, sheets or powders with very small cross-sectional dimensions.
A typical example of this early work was done by Tanner et al., see for example, U.S. Pat. Nos. 3,989,517 and 4,050,931, the disclosures of which are incorporated herein by reference. In these patents it was reported that amorphous ribbons (typically only 30 μm thick) could be made from Ti—Be, Zr—Be and Ti—Zr—Be systems at very high cooling rates of ˜106 K/s. Techniques suggested for use in forming amorphous alloys from these materials included, for example, splat quenching and melt spinning techniques. However, again the amorphous materials made from these alloys were limited by the size of the techniques to thin ribbons, sheet or powders. No bulk glass formers were ever identified in the binary systems or the ternary Ti—Zr—Be system, and indeed to date it is convention that such ternary beryllium alloys require cooling rates on the order of 106 K/s to maintain their amorphous properties.
Later studies tried to identify amorphous alloys with greater resistance to crystallization so that less restrictive cooling rates could be utilized, allowing in turn for the production of thicker bodies of amorphous material. The casting dimensions required to maintain the material in an amorphous state is referred to as the critical casting thickness. One class of materials that has garnered a great deal of attention over the past twenty years are bulk metallic glasses (BMG). These materials are noted for their high glass forming ability (GFA), good processability and exceptional stability with respect to crystallization. In addition these materials also exhibit high strength, elastic strain limit, wear resistance, fatigue resistance, and corrosion resistance. To date, families of binary and multi-component systems have been designed and characterized to be BMG if they readily form amorphous structures upon cooling from the melt at a rate less than 103 K/s. This low cooling rate allows for the fabrication of bulk parts with critical casting thicknesses formerly unattainable with traditional amorphous materials.
Prior research results teach that Beryllium bearing amorphous alloys require the presence of at least one Early Transition Metal (ETM) and at least one Late Transition Metal (LTM) in order to form BMGs. Indeed, it has long been believed that BMGs containing certain LTMs (e.g., Fe, Ni, Cu) have advantages including better glass forming ability, higher strength and elastic modulus, and lower materials cost. One exemplary set of bulk solidifying amorphous alloys are the highly processable Zr—Ti—Cu—Ni—Be BMGs (sold under the tradename Vitreloy® and disclosed in U.S. Pat. No. 5,288,344, the disclosure of which is incorporated herein by reference), which have been used commercially for a variety of items from sporting goods to electronic casings.
However, because of the high density of the LTMs used in these conventional BMGs, they have much higher densities than alloys excluding LTMs. For example, Vitreloy alloys have typical densities of ˜6 g/cc or above, and are therefore limited in their uses in structural applications, which usually require low density/high specific strength materials. For example, most structural metals, such as the conventional titanium alloys traditionally used in aerospace industries have a combination of high specific strength and low density. None of the prior art Ti-based LTM containing BMGs have material properties that compare to that of conventional titanium materials, such as, for example, pure titanium or Ti6Al4V alloy. For example, recently BMG forming alloys in the form of glassy ingots were discovered in the Ti—Zr—Ni—Cu—Be system. (See, e.g., F. Q. Guo, H. J. Wang, S. J. Poon, and G. J. Shiflet, Applied Physics Letters 86, 091907 (2005), the disclosure of which is incorporated herein by reference.) Amorphous rods with critical casting thicknesses up to 14 mm were successfully produced; however, for a typical Ti40Zr25Ni3Cu12 Be20 alloy, a density of ˜5.4 g/cc was obtained. This is much higher that the density of pure titanium, which is ˜4.52 g/cc.
Accordingly, it would be highly desirable to obtain a class of BMGs with a density on par with that of pure titanium or other conventional titanium based structural materials and the high strength, elastic strain Limit, wear resistance, fatigue resistance, and corrosion resistance properties of prior art BMGs. Such a class of materials would be particularly good for structural applications where specific strength and specific modulus are key figures of merit.